The geographical area covered by cellular mobile systems is divided into smaller radio coverage areas, cells, to achieve a more efficient utilization of the frequency band through frequency re-use. The size of cells varies from one mobile system to another, as well as geographically within a mobile system depending on a multitude of factors, such as the capacity required (number of channels, terrain, transmitting power levels, etc.). One factor which influences cell size is the maximum transmitting power of a mobile station. In the recent past, the mobile stations in conventional mobile systems were vehicle-mounted high-power radio stations whose transmitting power made large cell sizes possible. In addition to these, small hand-held mobile stations have emerged; these latter mobile stations have a considerably lower transmitting power than the vehicle-mounted stations in order to reduce current consumption, for example. These hand-held mobile stations have a considerably smaller range of operation and, as a result, the cell size must also be smaller.
In modern mobile systems, both low-power hand-held mobile stations and vehicle-mounted mobile stations that have a higher output power are used side by side. Further, two geographically overlaying types of cell, large macrocells and small microcells, have been employed. High-power mobile stations are thus assigned to a macrocell network, and low-power hand-held mobile stations to a microcell network. Because the high-power vehicle-mounted stations, moreover, move in the network with higher speed than the hand-held mobile phones, the use of macrocells results in a smaller number of handovers. The hand-held mobile stations are, in turn, relatively immobile and thus the number of handovers will not be unreasonable even in a dense microcell network. Decision on a handover of a mobile station between two microcells is made on the basis of radio connection, usually on the basis of the level or quality of the received signal.
In a hierarchical network divided to microcells and macrocells, a problem occurs regarding hand-held mobile stations that move fast in relation to cell size. As far as the network is concerned, a mobile station can be considered fast moving, if it is used, for example, in a moving vehicle such as train, tram, bus or car. In modern mobile systems, this kind of a fast moving hand-held mobile station may cause a considerable number of handovers, which may make controlling of the call very difficult. It would be advantageous if the fast moving mobile stations were served by macrocells, so that the number of handovers would not increase remarkably. On the other hand, slow moving hand-held mobile stations should be served by microcells so that macrocell capacity might be saved. For this reason it would be advantageous to be able to detect a fast moving hand-held mobile station in a microcell, and hand it up to an overlaying macrocell. In present systems, however, there are no means to detect a fast moving mobile station in a microcell network.